A fault tolerant server (Fault Tolerant Server: FT sever) that duplicates information processing of same data by performing same processing by each of two subsystems such as an online system and an offline system is known (see Patent Document 1). On this type of fault tolerant sever, a result of information processing performed by the online system is usually used, however, when failure occurs on the online system, the offline system performing the same processing replaces the online system as a new online system. Thereby, a stop of information processing can be prevented. Meanwhile, as a realization method of duplication processing, there is a hardware method and a software method.
Herein, as information processing is duplicated by using the two subsystems on the fault tolerant sever, content of a memory mounted on the each subsystem is coincident with each other. However, when an offline subsystem is in a condition of stopping while information processing is performed by an online subsystem, content of the memory of the online system and content of the memory of the offline system will be different each other. As a result, after exchanging a CPU of the offline system or starting an Operating System (hereinafter referred to as OS), it is required to copy collectively all pages of the memory of the online system into the memory of the offline system before synchronizing the offline system with the online system. Herein, a page of the memory is a unit to manage a storage region of the memory.    Patent Document 1: JP 2009-199197 A